The Outer Worlds

1. The Outer Worlds Draft 12/03/2006 Updated May 05, 2011

2. 2 1781: Uranus Discovered (accidentally!) by William Herschel using a 6 inch telescope [he thought it was a comet!]

3. 3 1846: Neptune Discovered • Le Verrier: proposed that another planet was creating deviations in orbit of Uranus. Predicts position. • John Couch Adams: independently does same • Johann Gottfried Galle: (student of Encke) using Le Verrier’s prediction, observes planet.

4. 4 Distance Comparison Sun Jupiter 12 year orbit Saturn 29 year orbit Uranus 86 year orbit Neptune 165 year orbit 1 AU 19.2 AU 30.1 AU 5.2 AU 9.6 AU 3.7% as much sunlight as Earth 1.1% as much sunlight as Earth 0.1% as much sunlight as Earth 0.3% as much sunlight as Earth

5. 5 Exaggerated Seasons On Uranus • Uranus’s axis of rotation lies nearly in the plane of its orbit, producing greatly exaggerated seasonal changes on the planet • This unusual orientation may be the result of a collision with a planetlike object early in the history of our solar system. Such a collision could have knocked Uranus on its side

6. 6 Uranus Rotates Sideways Seasons are 42 years long!

7. 7 Size Comparison Neptune Uranus 49,528 km (3.8 DE) 51,118 km (4.0 DE) 120,536 km (9.4 DE) 142,984 km (11 DE) 12,757 km (1 DE)

8. 8 Density Comparison • Masses determined from orbits of their moons • Uranus & Neptune are much higher density than expected, implies more heavier elements (INCONSISTENT with theories of planet formation which predict outer planets would be lighter in density) • Temperatures of Uranus & Neptune are nearly the same!

9. 1977 Rings Discovered 9

10. 10 1977 Voyager 2 Probe • 1986 Uranus Encounter • 1989 Neptune Encounter

11. 11 Sees thin, dark rings Uranus Neptune

12. 12 Voyager at Uranus and Neptune • Significant differences from Jupiter and Saturn • bluish color • Uranus very bland • Neptune faint belts and zones and also a few storms • much smaller (and less massive) than Jupiter and Saturn

13. 13 Uranus: Atmosphere • Nearly featureless, though recent Hubble images show some small storms • 82.4%H, 15.2% He, 2.3 % CH4 (methane) • The blue tint is caused by methane • Too cold for ammonia or water ice clouds, so any clouds are also methane • Does not emit excess heat

14. As the vernal equinox approaches, Uranus is becoming stormy: Heating of northern hemisphere produces storms, made visible by clouds of methane. 14 Recent Hubble Pictures

15. 15 Neptune: Atmosphere • Similar composition as Uranus (80% H, 19% He, 2% CH4) • More storms and clouds • Also belts and zones • why? less sunlight should mean less active atmosphere => Neptune, like Jup and Sat, emits more heat than it receives. This internal heat must drive atmospheric activity

16. 16 Neptune’s Clouds • Much more cloud activity is seen on Neptune than on Uranus • This is because Uranus lacks a substantial internal heat source

17. Neptune’s atmosphere is being warmed from below by internal heat. This extra heat drives large circular storms, like the “Great Dark Spot” seen above. 17

18. 18 Uranus & Neptune: Winds • Both Uranus and Neptune winds blow west at equator and east at N and S poles and mid-latitudes • Not understood why Neptune’s winds do not follow belts and zones like on Jupiter and Saturn • Rotation: 17 Hours Uranus, 16 Hours Neptune

19. 19 Uranus & Neptune: Magnetic Fields • No metallic layer in interior • fields generated by ionized liquid ammonia and water • Rotated large amounts from spin vectors and offset from center • perhaps currently undergoing reversal, as Earth does • perhaps very large impact knocked them off line

20. 20 Uranus & Neptune: Interior Structure • “Rocky” core ~ size of Earth • Liquid mantle – probably NH3 and H2O or related material • Liquid molecular H and He with some liquid methane • Thin atmosphere

21. Uranus+Rings+Moons 21

22. 22 Some of Uranus’s satellites show evidence of past tidal heating Uranus has five satellites similar to the moderate-sized moons of Saturn, plus at least 22 more small satellites. Biggest is 1600 km (i.e. half our moon’s)

23. Oberon (outermost moon) 23 • Old, heavily cratered, and icy surface. • little evidence of internal activity other than some unknown dark material that apparently covers the floors of many craters. • image shows several large impact craters towards the center of the image. • On the limb, a high mountain rises 6 kilometers (4 miles) above its surroundings. • There are bright rays similar to those seen on Jupiter's moon Callisto.

24. Titania (next closer) 24 • The LARGEST moon • Has a few large impact basins, but is generally covered with small craters and very rough rocks. • Long 1,600 km trench • large double walled crater can be seen towards the top of the image. • many faults indicating there has been internal forces molding its surface.

25. Umbriel 25 • Darkest of the Uranian moons • It is about the same size as Ariel and has about the same density. • The surface appears to be old with large craters and does not change much from one location to another. • Near the top is a puzzling bright ring called the fluorescent cheerio. It is probably the floor of a crater.

26. Ariel 26 • Brightest of the Uranian moons • It is about the same size as Umbriel and has about the same density. • The surface is pock-marked with craters • long rift valleys stretching across the entire surface. Canyons much like the ones on Mars appear in the pictures. The canyon floors appear as though they have been smoothed by a fluid. The fluid could not have been water because water acts like steel at these temperatures. The flow marks might have been made by ammonia, methane or even carbon monoxide

27. Miranda 27 • huge fault canyons as deep as 20 kilometers • Such a small moon should not have had any significant internal heating or evolution, so the features were quite a surprise. • scientists believe that Miranda may have been shattered as many as five times during its evolution

28. Miranda 28 • Gigantic Scarp!

29. Neptune has 13 known moons. 12 are small (< 500 km across) and irregular. One is the giant moon Triton. Proteus Triton Nereid 29

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31. 31 Triton is a frigid, icy world with a young surface and a tenuous atmosphere • Neptune has 13 satellites, one of which (Triton) is comparable in size to our Moon or the Galilean satellites of Jupiter • Triton has a young, icy surface indicative of tectonic activity • The energy for this activity may have been provided by tidal heating that occurred when Triton was captured by Neptune’s gravity into a retrograde orbit • Triton has a tenuous nitrogen atmosphere

32. 32 Triton: 38K, frozen methane & nitrogen

33. Triton has crater-free plains; perhaps the frozen calderas of extinct ice volcanoes? Density of Triton = 2100 km/m3; ice mantle over rocky core. 33

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36. Triton has geysers 5 miles high driven by vaporized nitrogen. Plumes are swept downwind in the very tenuous nitrogen atmosphere. 36

37. Triton is on a tilted retrograde orbit around Neptune. It has most likely been captured by Neptune. Tidal forces are moving Triton toward Neptune. In 100 million years, Triton will enter the Roche limit. 37

38. A farewell to the Solar System: Voyager 2 looks back at Neptune and Triton. 38

39. Notes • Polar caps are frozen nitrogen